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Contaminated water may have leaked from the disaster-struck Fukushima Daiichi nuclear power plant since April, the owner said Thursday.

Tokyo Electric Power Company Holdings Inc. said it is still unknown, due to a problem with monitoring equipment, whether radiation-contaminated water actually leaked from damaged reactor buildings.

In one of the world's worst nuclear disasters, the Nos. 1 to 3 units experienced fuel meltdowns while the No. 1, No. 3 and No. 4 units were also severely damaged by hydrogen explosions following a massive earthquake and tsunami in March 2011.

To prevent leakage of tainted water, Tepco has installed water-level indicators and kept groundwater levels in wells higher than the contaminated water levels inside the buildings.

The water level in the wells is usually kept a meter higher than the level of tainted water inside the buildings, according to Tepco.

But the water level in some wells has been found to be about 70 centimeters lower than indicated by the equipment, causing fears of possible leakage. The problem was discovered in wells that started operations between mid-April and early August.

The utility is now checking data on groundwater levels near the plant's Nos. 1 to 4 reactors.